Recently, electronic devices such as AV equipment, personal computers, and portable communications devices have been rapidly becoming cordless and more portable. As the power source for these electronic devices, alkaline storage batteries such as nickel-cadmium storage batteries and nickel-metal hydride storage batteries, and lithium ion secondary batteries are mainly used, since their reliability is high and their maintenance is easy. Also, these batteries are widely used in various other applications. In particular, they are increasingly used as the power source for electric assist bicycles, lawn mowers, electric vehicles, and the like, which require good characteristics at a large electrical load. Batteries for use as the power source for such devices are required to provide further large-current charge/discharge characteristics and safety, and the development of batteries meeting such requirements is necessary.
A common method for satisfying such requirements is to connect such batteries to form a battery pack or battery structure having good large-current discharge characteristics, so that such a battery pack or battery structure is used as the power source in the above-mentioned applications.
FIG. 11 illustrates an exemplary conventional battery structure. The battery structure of FIG. 11 is composed of two batteries 100A and 100B (cylindrical alkaline storage batteries), and the positive terminal of the battery 100A is connected to the negative terminal of the battery 100B by a bus bar 103.
The bus bar 103 is a substantially flat plate having a thin welded portion 103a at one end and a thick connecting portion 103b at the other end. The welded portion 103a is welded to a seal member 101 (positive terminal) of the battery 100A, while the connecting portion 103b is connected to the battery case bottom (negative terminal) of the battery 100B. The welded portion 103a is connected to the connecting portion 103b by a middle portion 103c, which is disposed between the welded portion 103a and the connecting portion 103b and has the same thickness as that of the connecting portion 103b. Also, the connecting portion 103b of the bus bar 103 is connected to the battery case bottom of the battery 100B by means of a negative terminal connector 104.
The negative terminal connector 104 is a thin, dish-shaped member having: a cylindrical portion 104a fitting to the outer face of the battery case bottom of the battery 100A; and a substantially circular flat plate 104b disposed so as to close one opening end of the cylindrical portion 104a. The flat plate 104b has an inner face facing the outer face of the battery case bottom, and an outer face opposite thereto. The negative terminal connector 104 is resistance welded to the outer face of the battery case bottom at welding spots 105 positioned on the wall of the cylindrical portion 104a. In FIG. 11, notches 106 between the welding spots 105 are provided to suppress idle current in welding.
The flat plate 104b of the negative terminal connector 104 has a pair of holders 108, which holds the connecting portion 103b of the bus bar 103 so as to join the negative terminal connector 104 and the bus bar 103. The holders 108 are formed by slitting and pressing the flat plate 104b of the negative terminal connector 104 so that the cross-section is L shaped. The connecting portion 103b is inserted between the pair of holders 108 and, in this state, the connecting portion 103b is welded to the holders 108 at welding spots 109 to join the bus bar 103 and the negative terminal connector 104. In this way, the electrodes of different polarities of the battery 100A and the battery 100B are connected (e.g., see Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-261083